It should not matter what size you are, nor your starting fitness, not even your flexibility. No doubt there are studios out there catering to pretending to do yoga while looking damn cute. But yoga is a spiritual discipline. It's about your internal experience, not the external appearance of your poses (and only in the U.S. is yoga *all* about poses). Meditation is a big part of yoga. Go for it!

A good instructor will adjust the poses to different person's needs.